This invention relates to stirring apparatus, more particularly stirring apparatus provided with a helical ribbon impeller or agitator capable of stirring at high effieiencies highly viscous liquids as in the case of preparing polyesters by polymerization and condensation reactions.
Generally, it is necessary to use stirring apparatus provided with impellers having suitable configurations determined by the physical characteristics of the liquid to be stirred, especially the viscosity thereof, so that it is usual to improve the efficiency of the stirring apparatus by using an impeller of adequate configuration.
Since the stirring and mixing of liquids having low viscocities are relatively easy there is no difficult problem except that when relatively large tanks are required. However, in the case of high visicosity liquids, uniform and rapid stirring and mixing are not always easy to obtain. Especially, if the constuction of the impeller were not adequate extremely non-uniform mixing would result which could not be eliminated by merely increasing the rotational speed of impeller or the mixing time. For this reason, where smaller impellers, turbine blades or propellers are used for admixing highly viscous liquids, dead spaces of substantial volume and number would be formed in the liquids.
Although anchor impellers are advantageous in that it is possible to accelerate the heat conduction from the inner surface of the tank to the bulk liquids and to prevent the build up of the solids deposited on the inner surface of the tank, the efficiency of the stirring and mixing is not always high. Furthermore, with the anchor impellers, although mixing of highly viscous liquids is possible, dead spaces are partially formed in the tank and the mixing is not so effective.
For the reasons described above, it has been recognized that helical ribbon impellers are suitable for mixing highly viscous liquids, but until today no helical ribbon impellers satisfactory for practical application has yet been available.